glgsg: Fix compile error with OpenGL ES 1

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -4314,7 +4314,9 @@ begin_draw_primitives(const GeomPipelineReader *geom_reader,
   }
   nassertr(_data_reader != nullptr, false);
 
+#ifndef OPENGLES_1
   _instance_count = _supports_geometry_instancing ? num_instances : 1;
+#endif
 
   _geom_display_list = 0;